1. Field of the Invention
The present invention relates to voltage limiters using field emission techniques to limit the voltage on radio frequency transmission lines.
2. Description of the Prior Art
Voltage limiters are used to protect delicate circuits from damage due to overvoltage caused by large voltage pulses. Conventional voltage limiters, also referred to as hardening devices, use semiconductor p-n junction devices to provide a conditional short circuit to ground. An effective hardening device, however, needs to provide extremely fast in-band limiting in order to limit the damaging effects of transient pulses.
The speed of a semiconductor device such as a metal semiconductor field effect transistor (MESFET) is limited by the time it takes for an electron to travel from the source to the drain. Impurity and phonon collisions within the lattice of the semiconductor material lead to electron velocity saturation.
Vacuum microelectronic technology has been investigated as an alternative to solid state electronics to provide more robust and higher-speed limiting. Vacuum valves have a cathode and an anode and pass electrons from the cathode to the anode uninterrupted by molecular collisions. This technique, referred to as ballistic transport, yields transit times of less than one picosecond for threshold voltages of 100 volts and dimensions on the order of 1 micrometer. However, complete switching times are impaired by parasitic capacitances, characteristic of small devices, as well as packaging capacitance.
A primary concern of any protection device is that it provides a very low impedance alternative path before the incoming transient pulse causes damage. FIG. 1 is a diagram illustrating the amount of energy supplied to a component before the protection device is turned on. The power P(t) of the transient pulse shown in FIG. 1 is not controlled by the protection device until time t=xcfx84. Thus, the component being protected suffers from spike leakage, defined as the energy Espike that leaks through the protection device from the time t=0 until the protection device is turned on at time t=xcfx84.
Existing semiconductor devices and vacuum valves have disadvantages as protection devices. Semiconductor devices typically rely on low-voltage transport in a low-density electron gas. Ionizing radiation can cause an excitation of carriers, changing the density of the electron gas, thereby leading to significant shifts in bias voltage. The result may be a transient upset, or permanent damage. Vacuum valves, however, use either a metal or highly-doped silicon cathode as the source of electrons, and therefore are more resistant to affects due to ionizing radiation. Nevertheless, vacuum valves operate at much higher voltages than semiconductors, making the vacuum valves far less sensitive to large voltage transients. Moreover, vacuum valves require an energy source, such as an electron gun or heater, to induce electron flow. Alternatively, vacuum valves may use a radioactive source such as tritium to provide seed electrons for a plasma discharge, such as disclosed in Patel et al., xe2x80x9cMicrostrip Plasma Limiterxe2x80x9d, 1989 IEEE MTT-S International Microwave Symposium Digest, Vol. III, pp. 879-882, Jun. 13-15, 1989, Long Beach, Calif., the disclosure of which is incorporated in its entirety by reference. However, vacuum valves using radioactive sources can be difficult to manufacture and handle under required radioactive protocols. Moreover, plasma discharges are destructive to internal components.
An improved vacuum valve was proposed in Glenn, xe2x80x9cPreliminary Investigation into the Development of Hardening Devices Using Vacuum Microelectronics Technology,xe2x80x9d HDL-PR-92-4, dated September 1992 and distributed by the Harry Diamond Laboratories under sponsorship by the U.S. Army Research Laboratory (hereinafter xe2x80x9cthe Glenn documentxe2x80x9d), the disclosure of which is incorporated in its entirety by reference. FIG. 2A shows a plane-to-plane cathode-anode geometry in a vacuum tube limiter, whereby the cathode emits electrons by thermionic emission: electrons are thermally excited from the cathode by a heater, modulated by a grid and collected at the anode.
According to the Glenn document, improved vacuum valve limiters can be developed using field emission instead of using thermionic emission as stated above. FIG. 2B shows a point-to-plane field emitter geometry, whereby field emission tips formed on the cathode have a radii of curvature on the order of angstroms, resulting in a high electron field at the cathode surface producing a more reliable electrical breakdown.
Although the Glenn document suggests the desirability that vacuum diode limiters be compatible with microwave circuitry, the Glenn document provides no disclosure or suggestion of the necessary specifications to make such a device. Moreover, the Glenn document does not recognize whether microwave limiters can be manufactured to satisfy limitations of microwave technology and the requirements of the limiter, namely a device having a fast response time and a relatively low threshold.
There is a need for a limiter that provides a wide bandwidth to prevent transmission line signals having a large voltage from damaging electronic components.
There is also a need for a voltage limiter that is able to operate in the microwave frequency ranges.
There is also a need for a microwave voltage limiter having a bandwidth including several octaves at a selected microwave frequency.
There is also a need for a passive voltage limiter that operates without externally supplied heat, voltage, or radioactive sources.
There is also a need for a voltage limiter that may operate either in a vacuum or with a gaseous medium between the cathode and the anode.
These and other needs are met by the present invention, which provides a voltage limiter having a large bandwidth (DC to millimeter waves) to prevent large voltages on a transmission line from destroying sensitive electronic components. The voltage limiter of the present invention uses field emitters to generate an electron flow above a predetermined breakdown threshold voltage. The use of field emitters enables generation of an electron flow without radioactive, thermal, or electrical sources typically used to provide xe2x80x9cseedxe2x80x9d electrons for a plasma discharge. Moreover, the voltage limiter of the present invention may be operated with a vacuum or gas within an airtight enclosure, allowing the predetermined breakdown threshold voltage and the voltage necessary to maintain the electron flow to be adjusted to desired values.
The present invention also comprises a microstrip transmission line that enables propagation of electromagnetic microwave energy over a bandwidth of multiple octaves. The microstrip electrically contacts a selected group of the field emitters. Since the field emitters are arranged as an array having a predetermined distribution per unit area, the width of the microstrip determines the number of field emitters used in the cathode, thereby affecting the total power handling capability of the voltage limiter.
Moreover, the region or space around the field emitter array and around the microstrip transmission line has a dielectric constant; also, the field emitter array and microstrip transmission line have impedances that minimize reflections between the boundary of the microstrip and the corresponding field emitter. An insulating layer supporting the field emitters has a known dielectric constant. The region between the cathode and the anode also has a known dielectric constant. Hence, the impedance of the voltage limiter can be determined for impedance matching with a transmission line.
Thus, the present invention provides a voltage limiter that has a known impedance that can be matched with a transmission line for voltages below the predetermined breakdown threshold voltage. If electromagnetic energy on the transmission line exceeds the predetermined breakdown threshold voltage of the dielectric medium between the selected group of field tips and the anode, the impedance of the voltage limiter is substantially reduced and an electron flow is formed between the selected group of field emitters and the anode, creating a reflection wave that interferes with the electromagnetic energy on the transmission line.
Thus, the voltage limiter of the present invention provides rapid response to high voltages for microwave signals on a transmission line over a multiple octave bandwidth, with sufficient flexibility to adjust the predetermined breakdown threshold voltage and the electron flow.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.